The magwell of the CZ 457 holds the magazine in the receiver. You can replace it with an aluminium magwell or buy a complete factory magwell:
Position 7 aluminium magwell kit: CNC machined 7075 aluminium with an anodized finish. It replaces the factory plastic magwell for better durability, feeding and faster reloads. Made for the CZ 457 in .22 LR.
CZ magwell complete: the factory assembled magwell, for correct alignment and secure retention of the magazine.